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Stereoelectronic effects of the arenophile (η5-Cp*) Ru+ in electron-rich σ-arylacetylide iron complexes including redox-driven haptotropic rearrangement: An approach toward readable binary molecular memory components

The organoiron vinylidene derivatives [(η2-dppe) (η5-Cp*)Fe=C=CH-(1-naphthyl)][X] (Cp* = C 5-Me5; dppe = 1,2-bis(diphenylphosphino)ethane; X = BPh4, 3[BPh4]; X = PF6, 3[PF6]) were synthesized from (η2-dppe)(η5-Cp*)FeCl and 1-naphthyl acetylene in the presence of NaBPh4 or NaPF6, respectively. The daughter organometallic acetylide complex, (η2-dppe)(η5-Cp*)Fe-C≡C-(1-naphthyl), 4, was obtained, in 81% yield, upon deprotonation of the vinylidene precursor, 3[BPh4], by t-BuOK in a MeOH/THF mixture at room temperature. One new dicationic heterobimetallic vinylidene Fe(II)-Ru(II) complex, 5[PF 6]2, was obtained, in 74% yield, upon reaction of the 1-naphthyl vinylidene precursor 3[PF6], with [(η5- Cp*)Ru(CH3CN)3][PF6]. Binuclear acetylides were prepared via η6 complexation of the (η5-Cp*)-Ru+ arenophile onto either the substituted or the free naphthyl ring of 1-naphthyl acetylide derivative 4, and both of these haptotropomers, 6A[PF6]·and 6B[PF6], were isolated. A third bimetallic model compound, 2[PF6], was prepared in 69% yield via (η5-Cp*Ru) coordination of the ethynyl phenyl ring of the known complex (η2-dppe) (η5-Cp*)Fe-C≡C-Ph (1). All three complexations are regioselective, occurring only on the acetylenic aryl moiety instead of the competing dppe phenyls. The thermally stable FeIII counterparts, 2[PF6]2, 4[PF6], and 6B[PF6] 2, were obtained (70-86% isolated yield) upon oxidation, in THF at -60°C, with ferrocenium hexafluorophosphate. All the new compounds were thoroughly authenticated using analytical and spectroscopic methods. In the heterobimetallic species, the aromaticity of the acetylide aryl linker is changed in situ via (i) the complexation of the (η5-Cp*) Ru+ arenophile and (ii) for the case of the 1-naphthyl substituent, by the η6-η6 inter-ring haptotropic migration of this group between naphthyl rings. The upshot is that these compounds exhibit significantly different degrees of electronic communication between the two organometallic termini across the length of the ethynediylaryl segment as a function of arenophile location: either (i) collinear to the Fe-C≡C wire-like segment (2[PF6] and 6A[PF6]) or (ii) on the free ring of the 1-naphthyl moiety (6B[PF6]). To establish the degree of electron transfer within the novel compounds reported herein, their physical properties are compared using NMR, UV-vis, IR, Moessbauer, and electron spin resonance (ESR) spectroscopies, as well as cyclic voltammetry and X-ray crystallography. Accordingly, the solid-state structures of all seven novel, electronrich organometallic acetylides are described, including both haptotropomers 6A[PF6] and 6B[PF6], Finally, the first redox-driven inter-ring haptotropic rearrangement of (η5- Cp*)Ru- between naphthyl rings was shown to occur.